It is a generally known requirement of certain computer graphic output devices to convert the resolution of digital input images prior to outputting them at a predefined output resolution (pixel density or raster density). Specifically, printer systems capable of printing image data and/or character data have become widely used. Furthermore, printer systems of this type currently provide high resolution 600 spots per inch (spi) output capabilities which, in many cases, is higher than the input resolution available. This results in a situation where the performance of the printer may not be used to the maximum. For example, many existing input scanners and personal computer software packages operate at a medium image resolution of 300 spi, resulting in either an incompatibility with higher resolution printers or underutilization of the output capabilities of the high resolution printers.
For these reasons, current technology is focused upon means for conversion of image data of one resolution to an image of a second resolution. It is a commonly known practice to implement this type of conversion through simple pixel and raster level operations, such as pixel doubling. Alternatively, more elaborate methods have been developed to facilitate this type of conversion, some of which are described below.
A particular problem arises in the resolution conversion of bit map images, where the images have been previously optimized for a specific printing technology. Thus, for example, images may be optimized for write-black printer technology, which tends to thicken lines, and subsequently transmitted to a write-white printer, which tends to thin lines. Such images will appear undesirably lighter. Single pixel lines will tend to be invisible. Another effect that might be present in an optimized image might be halfbitting, which may not be required in a higher resolution image
U.S. Pat. No. 3,096,398 to Gabor et al. discloses a method of supplementing the information received from a television broadcast system. The method of supplementation used involves interpolation between parts of the transmitted information to produce supplementary approximations of non-transmitted portions of a picture. A novel method of interpolation, contour interpolation, is taught wherein an apparatus is used to sense contours within selected picture portions and the interpolation to produce approximations to unselected picture portions is carried out both with regard to the position of such contours in the selected picture portions actually communicated and to their amplitudes.
U.S. Pat. No. 4,508,463 to Wang et al. teaches a technique and apparatus used with a dot matrix printer to provide high print resolution while printing at relatively high speeds. The higher print resolution is achieved through the use of a bi-directional printing mechanism including a method of offsetting the pin locations of the first printing pass with respect to corresponding pin locations in a pass of opposite direction. Furthermore, the technique taught includes a means of compact character encoding to reduce memory requirements within the printer system.
U.S. Pat. No. 4,632,579 to Takano et al. discloses a method of increasing print resolution by mechanically altering the relative position of the print head with respect to the paper during subsequent printing passes to increase the output resolution of the printer. Also disclosed is a means for controlling the activation of printing pins using a microprocessor based interpolation scheme to synthesize a secondary dot matrix pattern based upon a primary dot matrix pattern contained in memory for the associated character or pattern.
U.S. Pat. No. 4,679,039 to Nell et al. teaches a method for smoothing the display of contiguous parallel line segments in a manner to reduce the discontinuities that occur near the ends of the line segments. Smoothing is achieved by adding auxiliary dots of a lesser diameter below the larger main dots forming a first line segment in a given row and adding the same size auxiliary dots above the main dots of an adjacent line segment when the latter are in a row below the given row. The smoothing operation is optimized for multiple cases and line orientations and more than three different dot sizes can be used in the smoothing operation.
The above patents disclose methods of increasing the apparent resolution of an output device through a manipulation of the output device according to a predefined algorithm based on interpolation of adjacent image information. While these techniques are useful for the older technology printing and display devices, they are limited in application to the higher resolution output devices found in current computer graphics environments. Furthermore, the patents cited do not teach any methods for altering the interpolated output data to achieve a more desirable visual appearance.
U.S. Pat. No. 4,827,352 to Yoneda et al. discloses an image processing apparatus comprising a means for receiving a continuous-tone input image and converting the input into a binary image through the use of a dither matrix. Subsequently, the binary image is converted to an alternate pixel-density based upon a window whose size is related to the original dither matrix. The conversion is accomplished using a processing means to analyze a window of the first binary image to determine an average image density within the window and subsequently applying a second dither matrix, to generate a binary image at an alternate resolution.
U.S. Pat. No. 4,841,375 to Nakajima et al. teaches an image resolution conversion apparatus for converting the pixel density of image data, comprising an input means for receiving image data, a selection means to determine the appropriate pixel-density conversion ration, and a pixel-density converting means. The pixel-density converting means comprises, inter alia, a conversion-pixel density operating means for calculating a conversion-pixel density based on the input pixel data and a binary encoding means for subsequently applying a threshold to the conversion-pixel density value and generating a conversion-pixel binary output. Also disclosed is a threshold-value setting means which dynamically adjusts the threshold value to be applied within the binary encoding means on the basis pattern detection applied to the original or reference image data.
The Yoneda and Nakajima patents above deal with the current output printing technology and the requirements for a means to convert images to alternate resolutions. Specifically, both define methods of converting resolutions of previously binary encoded images using a means for determining an estimated grey density for a region of the binary image. Subsequently, the grey density is used as an input to a binary encoder, either a dither matrix or thresholding function, thereby generating a converted output image. The Nakajima patent further discloses a means for altering the threshold over a local region, to alter the characteristics of the converted image.
The Yoneda and Nakajima patents, utilize methods that approximate a grey density for a target area and subsequently encode the data to a converted binary format, potentially masking single pixels or adding structure to the converted image.